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A first humidified gas is supplied through a first supply port to a
sheet. A second humidified gas is supplied to a space where nozzles of an
inkjet recording head are exposed through a second supply port located at
a position closer to the recording heads than the first supply port, to
increase atmosphere humidity in the space. The sheet portion having a
moisture content increased by the humidification in advance is advanced
into the space having the increased atmosphere humidity, to record on the
sheet using the inkjet recording head. An amount of humidification with
the first humidified gas is set according to at least one recording
condition.

1. A method for recording on a sheet to be conveyed with a recording head
of an inkjet type in which nozzles are formed, the method comprising:
supplying a first humidified gas to the sheet with a first supply port;
supplying a second humidified gas to a space where the nozzles are
exposed, with a second supply port located at a position closer to the
recording head than the first supply port; recording in the space where
atmosphere humidity is increased, using the recording head, on the sheet
having a moisture content increased in the supplying the first humidified
gas step; and setting an amount of humidification of the first humidified
gas depending on at least one recording condition.

2. The method according to claim 1, wherein a plurality of the recording
heads are arranged along a conveying direction, and wherein at least a
part of the supplied second humidified gas flows along the direction
through a space including a gap between the nozzles of the plurality of
the recording heads and the sheet.

3. The method according to claim 1, wherein the recording condition
includes at least one of sheet type, sheet size, sheet thickness, sheet
conveying speed, coating on one or both side of sheet, and continuous or
discontinuous sheet form.

4. The method according to claim 1, wherein the recording condition is
whether the recording is made on a first side of the sheet or on a second
side of the sheet.

5. The method according to claim 1, wherein, at least one of a humidity
and a flow rate per unit time of the first humidified gas supplied
through the first supply port is set according to the recording
condition.

6. The method according to claim 1, wherein, a staying time of the sheet
in a humidifying area where the supplied first humidified gas is set.

7. The method according to claim 6, wherein, the staying time is set, by
forming a loop in the conveyed sheet in a non-humidifying area that is
located near the first supply port or between the first and second supply
ports, and changing a size of the loop according to the recording
condition.

8. The method according to claim 1, further comprising: setting an amount
of humidification with a humidified gas supplied with the second supply
port according to the recording condition.

9. A method for recording images on a sheet having a first and a second
side, using at least one inkjet recording head having nozzles,
comprising: supplying a first humidified gas to the first side of the
sheet, with a first supply port; supplying a second humidified gas to a
space where the nozzles are exposed, with a second supply port located at
a position closer to the recording head than the first supply port;
recording in the space where atmosphere humidity is increased, using the
inkjet recording head, on the first side of the sheet having a moisture
content increased; and after the recording reversing the sheet upside
down; after the reversing supplying the first humidified gas to the
second side of the sheet, with the first supply port; and recording in
the space, using the inkjet recording head, on the second side of the
sheet having the moisture content increased.

10. The method according to claim 9, wherein an amount of humidification
with the first humidified gas in the supplying the first humidified gas
in the first side of the sheet is different from an amount of
humidification with the first humidified gas in the supplying the first
humidified gas in the second side of the sheet.

11. The recording method according to claim 9, wherein the sheet portion
having recorded is heated for drying.

12. An apparatus, comprising: a recording unit including a recording head
of an inkjet type in which nozzles are formed; a first supply port for
supplying first humidified gas to a sheet to be conveyed; and a second
supply port for supplying second humidified gas to a space where the
nozzles are exposed, the second supply port is provided at a position
between to the recording head and the first supply port in a direction in
which the sheet is conveyed; and a control unit configured to control
setting of an amount of humidification with the humidified gas supplied
with the first supply port according to a recording condition.

13. The apparatus according to claim 12, wherein in the recording unit, a
plurality of the recording heads are arranged along the direction, and
wherein at least a part of the second humidified gas supplied with the
second supply port flows through a space including a gap between the
nozzles of the plurality of the recording heads and the sheet.

14. The apparatus according to claim 12, further comprising: a drying
unit configured to heat and dry the sheet after an image is recorded on
the sheet, and wherein the drying unit discharges a humidified gas with
high temperature and humidity, which is reused in generation of at least
one of the first and second humidified gases.

15. The apparatus according to claim 12, wherein the recording condition
includes at least one of sheet type, sheet size, sheet thickness, sheet
conveying speed, coating on one or both side of sheet, and continuous or
discontinuous sheet form.

16. The apparatus according to claim 12, wherein the recording condition
is whether the recording is made on a first side of the sheet or on a
second side of the sheet.

17. The apparatus according to claim 12, wherein, at least one of a
humidity and a flow rate per unit time of the first humidified gas
supplied through the first supply port is set according to the recording
condition.

18. The apparatus according to claim 12, wherein, a staying time of the
sheet in a humidifying area where the supplied first humidified gas is
set.

19. The apparatus according to claim 18, wherein, the staying time is
set, by forming a loop in the conveyed sheet in a non-humidifying area
that is located near the first supply port or between the first and
second supply ports, and changing a size of the loop according to the
recording condition.

20. The apparatus according to claim 12, further comprising: a set unit
for setting an amount of humidification with a humidified gas supplied
with the second supply port according to the recording condition.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a recording method and recording
apparatus capable of suppressing drying of ink in inkjet recording heads.

[0003] 2. Description of the Related Art

[0004] Japanese Patent Application Laid-Open No. 2000-255053 discusses a
method, in a printing apparatus in which inkjet recording heads are
fixedly arranged in a line in the direction of sheet conveyance, for
maintaining humidity of the inkjet recording heads and suppressing drying
of ink by continuously supplying humidified gas toward a nozzle of each
of the recording heads from the upstream side thereof.

[0005] A Sheet such as paper that has an equilibrium moisture content (the
state at which the sheet is neither gaining nor losing moisture) that
changes according to a humidity in the air: the sheet absorbs moisture at
higher humidity, and releases moisture at lower humidity. When a sheet is
fed to an area near recording heads, the area having a high humidity due
to humidified air supplied thereto, the sheet starts to absorb moisture.

[0006] As a result, some drop in humidity in the atmosphere occurs, and
may disturb appropriate moisturization of the recording heads. Especially
in a configuration with a plurality of recording heads fixedly arranged
in a line in the direction along which humidified air is introduced, it
takes some time for the humidified gas to flow down the line, and the
moisture is absorbed by the recording heads in the order arranged. This
tends to make the humidity retention of the downstream recording heads
insufficient.

[0007] In view of the above problem, a first object of the present
invention is to provide a recording method and recording apparatus
capable of maintaining recording heads at an appropriate moisture level
and suppress drying of ink during recording operation. The drying level
of ink during recording operation of a recording heads changes depending
on various conditions for the recording. For example, a sheet has a
different equilibrium moisture content depending on the type of the sheet
for the same ambient conditions. The sheets for inkjet printing are
generally formed of base paper such as resin coated paper (hereinafter,
referred to as RC sheet) and paper based paper (hereinafter, referred to
as baryta paper).

[0008] The RC paper is formed of base paper coated with resin, and absorbs
less moisture in the fiber of the paper than baryta paper. In other
words, different types of sheets have different moisture absorption
properties (different amount and rate of moisture absorption).
Accordingly, when a sheet having a moisture absorption property larger
than expected is used, since it takes a period of time for humidified gas
to flow down the line of recording heads, which is supplied from the
upstream side of the recording heads, and moisture in the gas is absorbed
by the sheet, the humidity of the recording heads on downstream side is
likely to become insufficient.

[0009] In view of the above problem, a second object of the present
invention is to provide a recording method and recording apparatus
capable of moisturizing recording heads without fail independently of the
recording conditions.

SUMMARY OF THE INVENTION

[0010] According to an aspect of the present invention, a method for
recording on a sheet to be conveyed with a recording head of an inkjet
type in which nozzles are formed includes supplying a first humidified
gas to the sheet with a first supply port, supplying a second humidified
gas to a space where the nozzles are exposed, with a second supply port
located at a position closer to the recording head than the first supply
port, recording in the space where atmosphere humidity is increased,
using the recording head, on the sheet having a moisture content
increased in the supplying the first humidified gas step, and setting an
amount of humidification of the first humidified gas depending on at
least one recording condition.

[0011] Further features and aspects of the present invention will become
apparent from the following detailed description of exemplary embodiments
with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate exemplary embodiments, features,
and aspects of the invention and, together with the description, serve to
explain the principles of the invention.

[0013] FIG. 1 illustrates hysteresis of sheets.

[0014] FIG. 2 illustrates a configuration of a recording apparatus
according to a first exemplary embodiment.

[0015] FIG. 3 is a system diagram illustrating a humidifying apparatus.

[0018] FIG. 6 is a flowchart illustrating a procedure to set and control a
target humidity of a first humidified gas based on a sheet type.

[0019] FIG. 7 is a flowchart illustrating a procedure to set and control a
target flow rate per unit time of a first humidified gas based on a sheet
type.

[0020] FIG. 8 is a flowchart illustrating a procedure to set and control a
target humidity and a target flow rate per unit time of a first
humidified gas based on a sheet type and an ambient temperature.

[0021] FIGS. 9A and 9B illustrate a configuration of a loop forming unit
and an operation for loop formation.

[0022] FIGS. 10A and 10B each illustrate a structure having a loop forming
unit in an area 2.

[0023] FIGS. 11A and 11B each illustrate a structure having a loop forming
unit in an area 3.

[0024] FIG. 12 is a flowchart illustrating a procedure to set and control
a staying time of a sheet based on a sheet type.

[0025] FIGS. 13A and 13B each illustrate a structure of a recording
apparatus for duplex printing according to a second exemplary embodiment.

[0026] FIG. 14 is a flowchart illustrating an operation sequence of a
duplex printing.

DESCRIPTION OF THE EMBODIMENTS

[0027] Various exemplary embodiments, features, and aspects of the
invention will be described in detail below with reference to the
drawings.

[0028] Prior to the description of exemplary embodiments of the present
invention, first, hysteresis of a sheet as a recording medium, such as
glossy paper for inkjet, is described. FIG. 1 illustrates hysteresis
between absorption and desorption of moisture. For paper such as glossy
paper for inkjet, the relationship between absorption and desorption of
moisture as a function of relative humidity is not linear. As the
atmosphere humidity changes from the point A to the point B, that is, as
the relative humidity increases, the glossy paper for inkjet absorbs
moisture in the atmosphere.

[0029] On the other hand, when the relative humidity decreases from the
point C to the point B, the glossy paper for inkjet still contains an
amount of moisture at the point D, which is larger than that at the point
B. In other words, when glossy paper for inkjet is exposed to relative
atmosphere humidity, the paper contains a larger amount of moisture when
the relative humidity is decreased than when the relative humidity is
increased.

[0030] In addition, the decrease in relative humidity from the point C to
the point D causes smaller amount of desorption of moisture from the
glossy paper for inkjet. Accordingly, when the paper is exposed again to
a relative humidity equal to or more than that at the point C, the paper
absorbs a less amount of moisture when the relative humidity at the point
D is increased than that when the relative humidity at the point B is
increased.

[0031] Thus, when a sheet is conveyed into a recording unit after the
sheet is forced to absorb moisture, any absorption of moisture by the
sheet in the recording unit can be restrained even when the unit is
maintained at high relative humidity.

[0032] As a result, even when the relative humidity in the atmosphere
around recording heads is increased to prevent evaporation of ink from
the recording heads, the increased relative humidity can be maintained
and the drying of ink can be suppressed, because absorption of moisture
by the sheet is restrained. The present invention is based on such
consideration.

[0033] FIG. 2 illustrates a configuration of a recording apparatus
according to a first exemplary embodiment of the present invention, with
the arrow indicating a flow of humidified gas. This exemplary embodiment
uses humidified air, but any humidified gas may be used other than air.
The sheet is conveyed downstream along the sheet conveyance path while
printing. At an arbitrary position in the sheet conveyance path where the
sheet is conveyed from feeding means to discharging means, a side toward
the feeding means is referred to as "the upstream side", and the opposite
side toward the discharging means is referred to as "the downstream
side".

[0034] The recording apparatus of this exemplary embodiment utilizes a
roll-to-roll system. A sheet feeding unit 41 unwinds and feeds a
continuous sheet 2. A winding rotary unit 42 rolls up the sheet after
recording is performed thereon by a recording unit 9. The sheet feeding
unit 41 in FIG. 2 includes one roll, but may includes a plurality of
rolls for selective feeding of a sheet.

[0035] The recording unit 9 has a housing illustrated by the dotted line
in FIG. 2, and a conveying mechanism and a recording unit are
incorporated in the housing as a unit. The conveying mechanism includes a
platen 7 for assistance of sheet support, and pairs of rolls, each pair
consisting of a driving roller 6 and a driven roller 5. The driving
rollers 6 are rotatably embedded in the platen 7, and rotated by a
driving source to convey a sheet.

[0036] The driven rollers 5 are supported by a support member (holder) 8,
and located at positions opposite to the driving rollers 6 respectively
with a sheet therebetween. Between the pairs of the driving rollers 6 and
the driven rollers 5, recording heads 1 are disposed as a recording unit.
The recording heads 1 are full-line inkjet recording heads that are fixed
and each have at least one nozzle to discharge ink in the width direction
of a sheet across the maximum width of the sheet for recording.

[0037] This exemplary embodiment is described using a thermal inkjet
printer, but is applicable to inkjet printers of piezoelectric element,
electrostatic element, and micro electro mechanical system (MEMS) types
for example.

[0038] The number of the recording heads 1 fixedly arranged in a line
along the sheet conveyance direction is equal to that of colors (six in
FIG. 2). The recording heads 1 are integrally supported by the supporting
member 8. To the recording heads 1, ink is supplied from an ink supply
unit (not illustrated) such as ink tanks. The recording heads 1 each may
be a unit combined with an ink tank that stores ink of a corresponding
color.

[0039] The recording unit 9 forms images in line printing process, by
applying ink of corresponding colors using the recording heads 1 to a
sheet while the sheet is moving. This exemplary embodiment is described
using a roll sheet that is a continuous sheet, but sheets in other form
may be used such as a continuous sheet that is folded into portions of a
unit length, and cut sheets.

[0040] A first humidifying unit 4 (first humidifying unit) is provided
upstream of the recording unit 9 along the sheet conveyance path. The
first humidifying unit 4 humidifies a sheet before the sheet is conveyed
to the recording unit 9. The first humidifying unit 4 supplies humidified
gas (a first humidified gas) to the sheet before the sheet enters the
recording unit 9, to increase moisture content of the sheets through
absorption of moisture.

[0041] The first humidifying unit 4 includes a humidifying apparatus, a
blower device, a supply port 43 (first supply port), and an intake port
44. A first gas in the first humidifying unit 4 is humidified by the
humidifying apparatus, and is emitted from the supply port 43 by the
blower device to be supplied to a sheet before the sheet enters the
recording unit 9 (a first humidified gas). The intake port 44 may be
provided at any position as long as it can take gas into the first
humidifying unit 4.

[0042] The intake port 44 is provided at a distance from the supply port
43 along the sheet, and the supply port 43 is disposed in such a manner
that the humidified gas is supplied therefrom in the direction
substantially parallel to the sheet. The gas supplied from the supply
port 43 can be suctioned into the intake port 44, and thereby circulation
of the humidified gas can be achieved to reduce the amount of water used
in the humidifying unit.

[0043] In addition to the first humidifying unit 4, a second humidifying
unit 3 (second humidifying unit) is provided to humidify the narrow space
where the nozzles of the plurality of recording heads 1 in the recording
unit 9 are exposed. The second humidifying unit 3 introduces humidified
gas (a second humidified gas) through a sheet entrance of the recording
unit 9, so that the atmosphere humidity in the narrow space where the
nozzles of the recording heads 1 are exposed is increased. This
moisturizes the nozzles of the recording heads, and suppresses drying of
the nozzles.

[0044] The second humidifying unit 3 is provided with the humidifying unit
that also operates for the first humidifying unit 4, a blower device, and
an intake port. The second humidifying unit 3 is connected to a supply
duct 46 having, at the distal end thereof, a supply port 45 (a second
supply port) to discharge humidified gas. The supply port 45 is located
near the sheet entrance of the recording unit 9, and supplies humidified
gas (the second humidified gas) to the narrow space in the recording unit
9.

[0045] The supply port 43 and the intake port 44 of the first humidifying
unit 4 are located upstream of the supply port 45 of the second
humidifying unit 3 relative to the recording unit 9. The gas humidified
in the second humidifying unit 3 is introduced to the supply port 45
through the supply duct 46, and thereby a gas humidifying unit of the
second humidifying unit 3 does not have to be located between the
recording unit 9 and the first humidifying unit 4.

[0046] The humidified gas supplied from the second humidifying unit 3
flows along the sheet conveyance path and the narrow space therearound in
the recording unit 9 from upstream to downstream. Specifically, around
the positions of the recording heads 1, the humidified gas flows through
the gap (hereinafter, referred to as a gap at recording) between the
front end (the surface having a nozzle) of each of the recording heads 1
and the sheet. Between adjacent recording heads 1, the humidified gas
flows through the gap between the support member 8 and the sheet.

[0047] In other words, the humidified gas passes through two different
gaps to the most downstream recording head 1. The recording gap is
usually only about 1 mm in the inkjet system. Thus, the flow rate of the
humidified gas is increased when passing through the recording gap, which
may adversely affect the impact precision of ink drops (i.e., main drops
and satellite drops) discharged from the recording heads 1 for recording.

[0048] Accordingly, the humidified gas from the second humidifying unit 3
is desirably set to have a flow rate of 1 m/sec or less at the recording
gap.

[0049] FIG. 3 is a system diagram illustrating the humidifying apparatus
that supplies humidified gas to the first humidifying unit 4 and the
second humidifying unit 3. The humidifying apparatus includes a mixing
unit 53 where outside air from an air intake 51 and the discharged gas
from a drying unit 52 are mixed to be a mixed humidified gas of an
appropriate temperature.

[0050] The drying unit 52 (not illustrated in FIG. 2) dries the sheet that
is wet with ink due to recording at the recording unit 9, before the
sheet is rolled up by the winding rotary unit 42. The drying unit 52
discharges the highly humidified gas of high humidity and temperature,
and part of the energy of the discharged gas is used to generate another
humidified gas, which increases the energy efficiency of the overall
apparatus. The humidifying apparatus further includes a humidifier 55 and
a water tank 54. The mixed gas sent from the mixing unit 53 is mixed with
water supplied from the water tank 54 to produce a humidified gas having
appropriate temperature and humidity to be supplied to the sheet.

[0051] The humidified gas generated in the humidifier 55 is temporarily
stored in a humidified gas tank 56. Then, for recording, the humidifying
unit is activated to send a necessary amount of the humidified gas to the
first humidifying unit 4 and the second humidifying unit 3 so that the
sheet can be humidified as required. Between the mixing unit 53 and the
humidifier 55, a heater is provided to finely control temperatures of the
mixed gas and the humidified gas.

[0052] Now, the humidity levels of the first humidified gas supplied from
the first humidifying unit 4 and the second humidified gas supplied from
the second humidifying unit 3 are described.

[0053] The humidity in the atmosphere around the recording heads 1 needs
to be at a level at which ink is unlikely to evaporate from the recording
heads 1. For example, for a temperature from 30 to 40.degree. C., the
corresponding relative humidity is about 60 to 70%. Thus, the second
humidifying unit 3 is desirably set to have a relative humidity of about
60 to 70%, but may have other relative humidity that suppresses the
evaporation of ink from the recording heads 1.

[0054] The first humidifying unit 4 desirably causes the sheet to absorb
moisture until it reaches the equilibrium moisture content. The maximum
moisture content is different depending on the sheet types. As a standard
value, the gas having an absolute humidity equal to or more than that of
the humidified gas supplied from the second humidifying unit 3 can be
supplied to the sheet from the first humidifying unit 4.

[0055] FIG. 4 is a block diagram illustrating a control system of an
inkjet recording apparatus according to the present exemplary embodiment.
Data such as characters and images to be recorded is input to a reception
buffer 11 of the inkjet recording apparatus from a host computer 10. Data
used to check errors in transfer of the data and to inform of operation
state of the inkjet recording apparatus are output to the host computer
10 from the inkjet recording apparatus.

[0056] The data in the reception buffer 11 is transferred to a memory unit
13 and temporarily stored in a random access memory (RAM) under control
of a central processing unit (CPU) 12. A mechanism control unit 14 drives
a mechanism unit 15 including line head carriages, caps, and wipers, in
response to commands from the CPU 12.

[0057] A sensor/switch (SW) control unit 16 sends signals to the CPU 12
from a sensor/SW unit 17 consisting of various sensors such as
temperature and humidity sensors, and switches. A display control unit 18
controls a display unit 19 such as a liquid crystal display, in response
to commands from the CPU 12.

[0058] The humidification control unit 20 controls a humidifying unit
(i.e., the first humidifying unit 4 and the second humidifying unit 3)
21, in response to commands from the CPU 12. In this control, the CPU 12
determines the amount of moisture to be supplied to the sheet, based on
various information such as ambient temperature, sheet type, sheet
thickness, temperature of line head, volume of input image data to be
recorded, to set humidifying conditions for the operation performed by a
humidifying unit 21.

[0059] A recording-head control unit 22 drive-controls the recording heads
1, detects state information of the recording heads 1 such as
temperature, and transmits the information to the CPU 12, in response to
commands from the CPU 12.

[0060] With the structure described above, the first humidifying unit 4 is
arranged upstream of the recording unit 9 along the sheet conveyance
path, so that the first humidified gas is supplied to the sheet before
the sheet enters the recording unit 9. This increases the moisture
content of the sheet before the sheet enters the recording unit 9. The
second humidifying unit 3 supplies the second humidified gas through the
sheet entrance in such a manner that the humidified gas flows from the
upstream to downstream along the conveyance path in the recording unit 9.

[0061] The second humidified gas is sent into the recording unit 9 in
advance to the sheet feeding, to increase the atmosphere humidity in the
narrow space where the nozzles of the recording heads 1 are exposed, and
to moisturize the nozzles. As a procedure, in a first step, the first
humidified gas is supplied through a first supply port to the conveyed
sheet to increase moisture content of the sheet.

[0062] In a second step, the second humidified gas is supplied to the
narrow space where the nozzles are exposed through a second supply port
located at a position closer to the recording heads than the first supply
port, to increase the atmosphere humidity in the narrow space, and
moisturize the nozzles. In a third step, the sheet portion having the
moisture content increased in the first step is advanced into the narrow
space having the atmosphere humidity increased in the second step, and
data is recorded on the sheet portion using the inkjet recording heads.

[0063] Through the steps, when the sheet passes through the space for
recording, the sheet already has the increased moisture content due to
the first humidified gas, thereby restraining the sheet from absorbing
the moisture in the second humidified gas.

[0064] As a result, the narrow space from the upstream recording head to
the downstream recording head is maintained at high humidity, which
reliably moisturizes the nozzles. Consequently, defective ink discharge
is prevented, such as discharge failure and discharge in wrong
directions.

[0065] FIG. 5 schematically illustrates areas between the first
humidifying unit 4 and the recording unit 9. The areas 1 to 4 are divided
based on the changes in moisture content of the sheet. In the area 1, the
sheet is unrolled prior to its entrance to the first humidifying unit 4.
The sheet in the area 1 has a moisture content Q1 that is determined by
the humidity and temperature in the housing of the recording apparatus.

[0066] In the area 2, the sheet is humidified by the first humidifying
unit 4 before recording. At the end of the area 2 humidified by the first
humidifying unit 4, the sheet has a moisture content Q2 larger than the
moisture content Q1 by an absorbed moisture content .DELTA.Q1 that was
sprayed by the first humidifying unit 4 to the sheet: Q2=Q1+.DELTA.Q1.
The absorbed moisture content .DELTA.Q1 varies depending on the
temperature and humidity of the humidified gas (a first humidified gas)
sprayed to the sheet in the area 2 and the period of time the sheet stays
in the area 2.

[0067] In the area 3, the moisture content of the sheet humidified in
advance in the area 2 is decreased. In the area 3, an amount of moisture
.DELTA.Q2 is desorbed from the sheet. As a result, at the end of the
humidifying area 3 immediately before the entrance to the recording unit
9, the sheet has a moisture content Q3 smaller than the moisture content
Q2: Q3=Q2-.DELTA.Q2. The amount of desorbed moisture .DELTA.Q2 varies
depending on the temperature and humidity in the housing of the recording
apparatus and the period of time the sheet stays in the area 3.

[0068] In the area 4, the humidified gas (a second humidified gas) is
supplied from the second humidifying unit 3 from upstream to downstream
in the space where the nozzles of the plurality of recording heads 1 are
exposed in the recording unit 9. The sheet in the temperature and
humidity in the area 4 has an equilibrium moisture content Q4.

[0069] If the sheet immediately before the entrance to the area 4 has a
moisture content Q3 approximately equal to the equilibrium moisture
content Q4, the sheet does not absorb moisture in the area 4, and thereby
the sheet causes no decrease in humidity around the nozzles. Thus,
controlling the moisture content Q3 not to be significantly smaller than
the equilibrium moisture content Q4 leads to appropriate moisturizing of
the nozzles of the recording heads 1.

[0070] From the above relationships Q3=Q2-.DELTA.Q2 and Q2=Q1+.DELTA.Q1,
Q3=Q1+.DELTA.Q1-.DELTA.Q2 can be obtained. In other words, the moisture
content Q3 is determined by the initial moisture content Q1, the absorbed
moisture content .DELTA.Q1 in the area 2, and the amount of desorbed
moisture .DELTA.Q2 in the area 3. Therefore, the ambient temperature and
humidity in the recording apparatus, the temperature and humidity of the
first humidified gas, and the periods of time the sheet stays in the
areas 2 and 3, are set so that the moisture content Q3 is approximately
equal to the equilibrium moisture content Q4.

[0071] The hysteresis of the sheet illustrated in FIG. 1 varies depending
on the type of the sheet. In the cases of a sheet having larger moisture
absorption property, the sheet may absorb much moisture in the area 4 to
disturb appropriate moisturizing of the recording heads 1, because the
relationship Q3=Q4 cannot be achieved but the equilibrium moisture
content Q4 exceeds the moisture content Q3: Q3<Q4. Accordingly, in the
present exemplary embodiment, according to at least one recording
condition such as a type of the sheet used, an amount of humidification
provided by the first humidified gas is variably set. There are several
approaches to variably set an amount of humidification, which will be
described below in sequence.

[0072] (Approach 1)

[0073] In a first approach to variably set an amount of humidification,
the humidity of the first humidified gas generated by the first
humidifying unit 4 is variably controlled. A higher humidity of the
humidified gas results in a larger amount of humidification. The
flowchart in FIG. 6 illustrates a procedure to set and control a target
humidity of the first humidified gas based on a sheet type.

[0074] In step S11, before recording, information about a sheet type is
obtained using a sensor that detects the sheet type. As an example of the
sensor, a medium sensor is known which optically reads a sheet surface
using an optical sensor to determine a type of the sheet based on the
obtained surface state information. Alternatively, the information about
a sheet type may be obtained using sheet information specified by an
operator, without using a sensor. Otherwise, the information about a
sheet type may be obtained from information in a file that stores
information about recording jobs.

[0075] The other known processes may be used. The following example is
described when the sheet type is either glossy paper or semi-glossy
paper. In this example, the glossy paper has baryta paper base, while the
semi-glossy paper has RC paper base.

[0076] In step S12, according to the sheet type information obtained in
step S11, a target temperature and humidity of the first humidified gas
generated by the first humidifying unit 4 is set with reference to a data
table stored in a memory in a control unit. Table 1 is a specific data
table with parameters. Since the baryta paper has larger moisture
absorption property than the RC paper, the glossy paper has a larger
target humidity than the semi-glossy paper. The target temperatures are
equal to each other in this example, but may be different.

[0077] In step S13, according to the target temperature and humidity set
in step S12, the first humidifying unit 4 is controlled. Specifically,
the humidification control unit 20 controls the humidifier 55 in FIG. 3
to variably generate humidified gas. Alternatively, while the preliminary
humidification at the first humidifying unit 4 is controlled, an amount
of humidification with the humidified gas supplied from the second
humidifying unit 3 (the supply port 45) may be controlled to be variably
set.

[0078] In the above sequence, since the glossy paper having larger
moisture absorption property can be also sufficiently humidified in
advance, the nozzles of the recording heads 1 are all appropriately
moisturized during recording operation. In the case where semi-glossy
paper is used, the amount of water consumed in the first humidifying unit
4 is small, thereby suppressing the electricity consumption in waste for
generation of humidified gas, and decreasing the frequency to refill
water for further reduction in running cost.

[0079] (Approach 2)

[0080] In a second approach to variably set an amount of humidification, a
flow rate of the first humidified gas per unit time supplied from the
first humidifying unit 4 is variably controlled. A larger flow rate
results in a larger amount of humidification. The flowchart in FIG. 7
illustrates a procedure to set and control a target flow rate of the
first humidified gas per unit time based on a sheet type.

[0081] In step S21, as in step S11 of FIG. 6, information about a sheet
type is obtained. In step S22, according to the information about the
sheet type obtained instep S21, a target flow rate of the humidified gas
per unit time supplied from the first humidifying unit 4 is set with
reference to a data table stored in the memory in the control unit. Table
2 is a specific data table with parameters. Since the baryta paper has
larger moisture absorption properties than the RC paper, the target flow
speed (which is proportional to flow rate) is set to be larger for the
glossy paper than that for the semi-glossy paper to change a flow rate.
The flow rate of the supplied humidified gas is change, but the
temperature and humidity of the humidified gas are maintained constant
(e.g., 30.degree. C. and 85%).

[0082] In step S23, according to the target flow rate set in step S22, the
first humidifying unit 4 is controlled. A change in flow rate per unit
time is achieved by changing the capability of a blower device in the
first humidifying unit 4, or changing the opening area formed in the
supply port 43. The blower device capability or the opening area is
variably controlled by the humidification control unit 20.

[0083] In the approach 2, the same effect as that of the approach 1 can be
obtained. As compared to the approach 1, advantageously, the time
required to change the amount of humidification is reduced. Thus, in the
case where the sheet feeding unit 41 includes a plurality of rolls to
selectively feed a sheet, change in the amount of humidification can be
switched in a short period of time when the sheet roll to be used is
switched.

[0084] (Approach 3)

[0085] In a third approach to variably set an amount of humidification, a
humidity of the first humidified gas generated by the first humidifying
unit 4 is variably controlled, and also a flow rate of the first
humidified gas per unit time supplied from the first humidifying unit 4
is variably controlled. Furthermore, these target values are changed in
response to an ambient temperature. The flowchart in FIG. 8 illustrates a
procedure to set and control these target values.

[0086] In step S31, like in step S11 of FIG. 6, information about a sheet
type is obtained. In step S32, the information about the ambient
temperature in the area where the sheet passes in the recording apparatus
is obtained, based on the output of a temperature sensor located in the
recording apparatus.

[0087] In step S33, according to the obtained information about the sheet
type and the ambient temperature, a target temperature and humidity of
the first humidified gas generated by the first humidifying unit 4 are
set with reference to a data table stored in the memory in the control
unit. In step S34, according to the obtained information about the sheet
type and the ambient temperature, a target flow rate of the first
humidified gas per unit time supplied from the first humidifying unit 4
is set with reference to a data table stored in the memory in the control
unit.

[0089] The parameters in Tables 3-1 and 3-2 are set so that the amount of
humidification is larger in a high temperature environment (at an ambient
temperature of 25.degree. C. or more) than in a low temperature
environment (at an ambient temperature less than 25.degree. C.). This is
because that the initial moisture content Q1 in the area 1 and the amount
of desorbed moisture .DELTA.Q2 in the area 3 in FIG. 5 vary depending on
an ambient temperature.

[0090] A lower ambient temperature results in a larger initial moisture
content Q1 and a smaller amount of desorbed moisture .DELTA.Q2. To
control the moisture content Q3 of the sheet before the entrance to the
recording unit 9 not to be significantly smaller than the equilibrium
moisture content Q4, higher preliminary humidification at the first
humidifying unit 4 is required. Accordingly, the amount of humidification
to the sheet is set to be larger in the higher temperature environment
(at an ambient temperature of 25.degree. C. or more).

[0091] In step S35, according to the target humidity and target flow rate
set in step S22, the first humidifying unit 4 is controlled. Depending on
the ambient temperature, not both but only one of the target humidity and
the target flow rate may be variable.

[0092] According to this exemplary embodiment, in addition to the above
effect, advantageously, parameters can be changed quickly in response to
a change in ambient temperature during recording operation, resulting in
reliable moisturization of the recording heads 1.

[0093] (Approach 4)

[0094] In a fourth approach to variably set an amount of humidification,
the period of time the sheet stays in the humidifying area (area 2) in
the first humidifying unit 4 is variably controlled. As a specific device
to change a staying time of a sheet, a loop forming unit 60 is provided
to form a loop in the sheet during its conveyance near the supply port
43, so that a change in the size of the loop changes a substantial
staying time of the sheet in the humidifying area. A larger loop results
in a longer staying time and a larger amount of humidification.

[0095] FIGS. 9A and 9B illustrate a configuration of a loop forming unit
and an operation for loop formation. FIGS. 10A and 10B each illustrate a
structure having a loop forming unit in the area 2 (a humidifying area in
the first humidifying unit 4).

[0096] A loop forming unit 60 includes a loop forming roller 65 and two
moving rollers movable around the loop forming roller 65. The loop
forming roller 65 and the two moving rollers 66 do not have any driving
force, and are driven to rotate. The sheet passes between the loop
forming roller 65 and the moving rollers 66. Conveyance rollers 61 are
disposed upstream and downstream of the loop forming unit 60 respectively
along the direction of sheet conveyance. The loop forming roller 65 is
moved vertically by a moving mechanism.

[0097] FIG. 9A and FIG. 10A illustrate the loop forming roller 65 located
at an upper position to make the sheet pass through without loop
formation. On the other hand, FIG. 9B and FIG. 10B illustrate the loop
forming roller 65 located at a lower position to form a loop in the
sheet.

[0098] As the loop forming roller 65 moves downward, the two moving
rollers 66 moves as if they escape laterally. The sheet wraps around the
lower half of the loop forming roller 65, and is also nipped between the
loop forming roller 65 and the moving rollers 66. The depression of the
sheet by the loop forming roller 65 corresponds to an increase in a
length of the sheet conveyance path. The increased length of the sheet is
referred to as "amount of loop".

[0099] FIG. 12 is a flowchart illustrating a procedure to set and control
a staying time of a sheet based on a sheet type. In step S41, like in
step S11 of FIG. 6, information about a sheet type is obtained.

[0100] In step S42, according to the information about sheet type obtained
in step S11, a target amount of loop to be formed using the loop forming
unit is set with reference to a data table stored in the memory in the
control unit. Table 4 is a specific data table with parameters. The
target amount of loop for glossy paper is larger than that for
semi-glossy paper, because baryta paper has larger moisture absorption
properties than RC paper.

[0101] In step S43, according to the target amount of loop set in step
S42, the loop forming unit is controlled. The mechanism control unit 14
in FIG. 4 controls the motor driving of the loop forming unit in the
mechanism unit 15.

[0102] As described above, an amount of loop of the sheet, which is a
length of the sheet conveyance path, can be changed in the humidifying
area of the first humidifying unit 4. A larger length of the sheet
conveyance path results in a longer staying time of the sheet in the
humidifying area, and in turn a larger amount of humidification with the
first humidified gas. As illustrated in Table 4, the glossy paper having
larger moisture absorption property is also sufficiently humidified in
advance, and thereby the nozzles of the recording heads 1 are all
appropriately moisturized during recording operation.

[0103] An amount of humidification can be controlled only by forming a
loop. Accordingly, as compared to the approach 1, advantageously, the
time required to change an amount of humidification can be further
reduced. When a continuous sheet is used, the loop forming unit located
upstream of the recording unit contributes to increase stability of
accuracy in conveying the sheet in the recording unit, leading to high
recording accuracy.

[0104] (Approach 5)

[0105] An approach 5 is a modification of the approach 4. FIGS. 11A and
11B illustrate a loop forming unit located in the area 3 (i.e., a
non-humidifying area between the first supply port and the second supply
port). The configuration and operation of the loop forming unit are
identical to those described with reference to FIG. 9. FIG. 11A
illustrates the loop forming roller 65 located at an upper position, and
FIG. 11B illustrates the loop forming roller 65 located at a lower
position to form a loop in a sheet.

[0106] In the area 3 that is a non-humidifying area, moisture is desorbed
from the sheet. Thus, a longer staying time of the sheet in the area 3
results in a larger amount of desorption of moisture, decreasing a
moisture content of the sheet. If the humidification in advance in the
first humidifying unit 4 is excessive, the loop forming roller 65 in the
area 3 can be used to form a loop in the sheet to cause desorption of the
excessive moisture. In this way, a sheet of any type can enter the
recording unit 9 with an appropriate moisture content Q3.

[0107] In the above exemplary embodiment, a preliminary humidification is
controlled based on a sheet type. The humidification can be influenced by
recording conditions other than sheet type. Examples of the conditions
include sheet size, sheet thickness, coating on one or both side of a
sheet, and continuous or discontinuous sheet form.

[0108] As for sheet size, a larger sheet size (area) increases an amount
of moisture required to cause the entire sheet to reach its equilibrium
moisture content. As for sheet thickness, a larger thickness increases an
amount of moisture to be supplied. As for coating, a sheet having coating
on both sides requires a larger amount of moisture than a one-side coated
sheet.

[0109] As for sheet form, a continuous sheet without boundary requires a
larger amount of moisture than cut sheets. Therefore, desirably these
recording conditions are also considered in variably setting the amount
of humidification with the first humidified gas.

[0110] The other recording conditions include a speed to convey a sheet
during recording. A higher sheet-conveyance speed reduces a staying time
of the sheet in the area 2, which may cause the sheet to enter the
recording unit 9 before the equilibrium moisture content is not reached.
In the case where the sheet-conveyance speed in the recording unit 9 is
not regulated, desirably, the amount of humidification with the first
humidified gas is variably set in response to the speed, as described
above.

[0111] In the case, any of the above approaches illustrated in FIGS. 6 to
8 and FIG. 12 may be used to change an amount of humidification.
Information about one of the recording conditions is obtained, instead of
information about sheet type. Alternatively, two or more recording
conditions may be combined as the information, so that the amount of
humidification is variably set.

[0112] The above example uses a line printer having full-line inkjet
recording heads that are fixed. The present invention is applicable to
serial printers as well as line printers. In a serial printer, images are
formed by alternate operations of scanning by recording heads and feeding
a predetermined amount of sheet in a step.

[0113] The number of scanning (passes) by recording heads per step may be
one or more depending on a recording mode. The amount of sheet movement
per unit time, which is the average sheet conveying speed in a step,
varies depending on the number of passes.

[0114] The number of recording passes substantially determines a sheet
conveying speed: a larger number of passes decreases a sheet conveying
speed. In a serial printer, since the sheet conveyance is stopped during
scanning by the recording heads, the staying time of the sheet in the
humidifying area of the first humidifying unit 4 varies depending on a
recording mode (the number of passes). Accordingly, desirably, based on a
sheet conveying speed as a recording condition, the amount of
humidification with the first humidified gas is variably set, as
described above.

[0115] A recording apparatus according to a second exemplary embodiment of
the present invention is described. In the above exemplary embodiment,
the amount of preliminary humidification with the first humidified gas is
variably set based on a sheet type. On the other hand, in the present
exemplary embodiment, a recording apparatus for duplex printing on a
continuous sheet is used, in which the amount of preliminary
humidification differs between recordings on a front surface (first side)
and a rear surface (second side).

[0116] FIGS. 13A and 13B illustrate a structure of an entire recording
apparatus for duplex printing according to the second exemplary
embodiment. FIG. 13A illustrates an operation in a first-side recording
mode to record a plurality of images in sequence on a first side of a
continuous sheet. FIG. 13B illustrates an operation in a second-side
recording mode to record a plurality of images in sequence on a second
side of the continuous sheet.

[0117] The recording apparatus of the present exemplary embodiment has the
same structure from a sheet feeding unit 41 to a recording unit 9 as
those of the above exemplary embodiment, and has areas 1 to 4. The
recording apparatus further includes a cutter unit 31, a drying unit 32,
and a winding rotary unit 33. The winding rotary unit 33 serves as a
reversing device that reverses two sides of a sheet upside down through
temporal roll-up of the sheet.

[0118] The recording apparatus includes a humidifying apparatus similar to
that described with reference to FIG. 3, except a drying unit 32 instead
of the drying unit 52 in FIG. 3. The drying unit 32 discharges heated and
humidified gas, which is reused in generation of humidified gas, thereby
increasing the energy efficiency of the overall apparatus.

[0119] FIG. 14 is a flowchart illustrating an operation sequence of duplex
printing. The sequence is executed under control of a control unit. In
step S51, a sheet is supplied from a sheet feeding unit 41. The sheet is
a continuous sheet with a first side facing upward.

[0120] In step S52, the first side of the sheet is humidified in advance
by a first humidifying unit 4. As described in the first exemplary
embodiment, an amount of humidification with a first humidified gas is
variably set according to a sheet type used or the other recording
conditions.

[0121] In step S53, a plurality of images are recorded in sequence on the
first side of the sheet that is conveyed to a recording unit 9 after the
preliminary humidification. The recorded images pass through the cutter
unit 31 in the state of the continuous sheet to the drying unit 32, where
the sheet portion with images are heated for drying. The drying unit 32
includes a blower that blows heated gas to the sheet.

[0122] In step S54, the sheet that has passed through the drying unit 32
is rolled up by the winding rotary unit 33 in the continuous form. For
the roll-up, the winding rotary unit 33 rotates counterclockwise (FIG.
13A). The operations in step S51 to step S54 are repeated until a
predetermined number of images are recorded or the entire sheet is used
up.

[0123] When a predetermined number of images are recorded, the sheet is
cut at the position behind the image recorded last by the cutter unit 31.
The continuous sheet downstream of the cut position is all rolled up by
the winding rotary unit 33. Simultaneously, the remaining continuous
sheet located upstream of the cut position is back fed to be rolled up by
the sheet feeding unit 41. The recording on the first side of the sheet
is done here. Then, recording on the second side of the sheet is started.

[0124] In step S55, the winding rotary unit rotates in the opposite
direction, that is, clockwise (FIG. 13B), so that the rolled-up sheet is
supplied again to the area 1 as a continuous sheet. The sheet is conveyed
upside down with the second side facing upward.

[0125] In step S56, the second side of the sheet is humidified in advance
by a first humidifying unit 4. The amount of humidification with a first
humidified gas is set to be different from that for the first side in
step S52. To change the amount of humidification, any one of the approach
1 to 5 in the first exemplary embodiment may be used.

[0126] In step S57, a plurality of images corresponding to the images on
the first side are recorded in sequence on the second side of the sheet
that is conveyed to a recording unit 9 after the preliminary
humidification. In step S58, the sheet is cut by the cutter unit 31 for
each of the recorded image.

[0127] The cut sheets pass through the drying unit 32 for quick drying. In
step S59, the cut sheets that have passed through the drying unit 32 are
discharged one by one to the outside of the recording apparatus. In this
way, a plurality of cut sheets having images on both sides are obtained,
and the operation sequence for duplex printing ends.

[0128] In the above sequence, the sheet rolled up by the winding rotary
unit 33 with recorded images on the first side is in the state after the
processes for preliminary humidification, recording (application of ink),
and drying are completed. Accordingly, the sheet has a moisture content
different from the initial moisture content at the sheet feeding unit 41.
The sheet of a desired thickness may have different moisture contents on
the first and second sides thereof.

[0129] The moisture content on the second side is decreased when the sheet
has passed through the drying unit 32 of high temperature after the
first-side recording. In other words, the moisture content on the second
side of the sheet in the second-side recording mode is likely to be lower
than that of the first side in the first-side recording mode.

[0130] Accordingly, in the second preliminary humidification in step S57,
the amount of humidification is increased as compared to that in the
first preliminary humidification in step S52, to minimize the difference
in the moisture content Q3 of the sheet between in the first-side
recording mode and in the second-side recording mode, before the entrance
to the recording unit 9

[0131] The above operation is effective when the drying unit 32 has an
adequate capability for drying. The capability of the drying unit 32,
however, may be very small, or the drying unit 32 may be eliminated in
some structures. In these cases, the sheet having recording on the first
side is inevitably rolled up by the winding rotary unit 33 without
adequate drying.

[0132] When the sheet is supplied for the second-side recording, the
moisture content of the second side of the sheet in the second-side
recording mode is likely to be higher than that of the first side in the
first-side recording mode. Thus, in the second preliminary humidification
in step S57, the amount of humidification is decreased as compared to
that in the first preliminary humidification in step S52, to minimize the
difference in the moisture content Q3 of the sheet between in the
first-side recording mode and in the second-side recording mode, before
the entrance to the recording unit 9.

[0133] As described above, the amount of preliminary humidification is set
to vary depending on whether the first-side recording mode or the
second-side recording mode, in duplex printing. The relative size of the
amounts of humidification is determined by presence/absence of the drying
unit 32, its capacity for drying, and the other conditions unique to the
apparatus. Consequently, in either mode, the nozzles in the recording
heads are appropriately moisturized, and high quality images can be
formed on the both of the first and second sides.

[0134] The benefit of both of the exemplary embodiments is that
humidification of a sheet in advance suppresses moisture absorption by
the sheet during recording, maintains recording heads at an appropriate
moisture level, and prevents defective discharge of ink. In the
humidification, the amount of humidification with humidified gas is
variably set depending on recording conditions, resulting in more
reliable moisturization of the recording heads.

[0135] While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all modifications, equivalent structures, and functions.

[0136] This application claims priority from Japanese Patent Application
No. 2010-131255 filed Jun. 8, 2010, which is hereby incorporated by
reference herein in its entirety.